Mine is back too and it sucks! Actually, I don't think that mine never really went away 100%. I've never had more than a few days break from it. In the first trimester, it seemed to be very erratic and unpredictable. Now I can somewhat control it as long as I pay attention to my body. Here are some things that really seem to make a big difference for me:
1) Never go too long without eating something. An empty stomach makes it worse
2) Make sure you are drinking enough fluids. Whenever I go too long without drinking, it's worse
3) Avoid strong odors or prolonged exposure to any odor
4) Take Vitamin B6 at every day. I take 25 mg 2x/day or 50mg 1x/day at bedtime if I forget the morning dose. As soon as I stop taking it, the nausea comes back.
It is disappointing and frustrating. I figured by this time (16+ weeks), I'd be feeling no MS and full of energy - that is definitely not the case. Just keep telling yourself that its temporary and it will all be worth it in the end! 
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